1. Field of the Invention
This invention is related to a light diffusing device particularly used for an edge lighted panel. This type of light diffusing device receives light from a source and provides a plane of light to be used for uniform illumination of a relatively large area. It is practically employed as an illumination for display devices such as advertising lamps, and further, as rear face illumination means for liquid crystal display devices.
2. Discussion of Background
Light diffusing plates such as frosted glass plates or opaque white glass plates have been widely employed as light diffusing devices. They are usually utilized with a light source disposed by a predetermined distance on the rear side of the light diffusing plate.
Because miniaturization of the light diffusing device is particularly in demand with respect to liquid crystal display devices and the like, the miniaturization of the illumination means is accordingly also desirable. In view of the requirements for miniaturization, the disposal of a light source in the vicinity of the edge face of the diffusing plate has been often used, with this particular edge face being utilized as a light incident face. However, uniform brightness over the entire surface of the diffusing plates cannot be obtained by merely disposing the optical source at the edge face of the frosted glass or opaque white glass plate.
In view of the above lack of uniform brightness, by the mere placement of the diffusing plate, it has been proposed, for instance, in U.S. Pat. No. 4,059,916 that matting fabrication be applied to the rear face in a hairline manner from the edge face on the side of the light source to an end face opposing that edge face. This provides for directionality for the optical transmission within the diffusing plate and further provides that the thickness of the diffusing plate is gradually decreased toward the opposing end face to thereby improve the brightness of the diffusing plate at the opposing end face in order to obtain a uniform brightness over the entire surface.
It is to be noted that one of the disadvantages of this type of structure shown in U.S. Pat. No. 4,059,916, is that it requires a very fine and delicate fabrication to the surface of the transparent substrate which of course provides problems with respect to the production and cost. Thus, this particular solution is unsatisfactory in the light of the present demand for cost reduction of the device.
A further attempt to solve the above problems concerns the use of light diffusing plates in which a light permeable light diffusing layer is formed over the entire surface of a transparent base plate with the exception of a light incident edge face of the base plate. A light diffusing layer is further formed over the entire light diffusing layer on the surface of the transparent base plate except for the face on which light is irradiated. This is disclosed in Japanese Unexamined Utility Model Publication (Kokai) No. 60-26001 (1985).
Another attempt at solving the problems in this area is indicated in Japanese Unexamined Utility Model Publication (Kokai) No. 60-26002 (1985) which provides a reflection layer formed on the rear face of a transparent base plate and a light permeable light diffusing layer formed over the entire surface of the transparent base plate with the exception of the rear face and a light incident face. Furthermore, a light screening layer is formed over the entire light diffusing layer on the surface of the transparent base plate with the exception of the face from which light is irradiated.
Although these attempts have provided a considerable improvement with respect to some of the above discussed disadvantages, it has been found that a portion of the incident light is reflected once again at the light diffusing face and absorbed by the light screening layer to produce optical losses.